Interaction of wall-bounded turbulence with flexible roughness

Interaction of wall-bounded turbulence with flexible roughness is investigated by direct numerical simulation (DNS) in turbulent channel flow with surface roughness elements consisting of rigid or flexible filamentous structures, uniformly implanted on both channel walls. DNS studies were performed using a lattice Boltzmann, Immersed-Boundary (LB-IB) method, employing a D3Q19, single relaxation time, BKG lattice model. A direct forcing IB scheme, employing reciprocal interpolation-spreading of operators, was adopted for the immersed-boundary scheme. The dynamics of the filaments was tracked by solving the Euler-Bernoulli equation, in which the inertia, tension, bending, and interaction forces were balanced. Simulations were performed in turbulent channel flows at a bulk Reynolds number of Re_b=7,200, corresponding to a friction Reynolds number of Re_τ0≈ 221 in a `base' turbulent channel flow with smooth, no-slip walls. Filamentous structures of height and spacings ranging from 4 to 16 in base flow wall units, with a density ratio of ρ_solid/(ρ_fluid ε_solid)≈100, where ρ_solid and ε_solid are the linear density and hydrodynamic area of the filaments, were investigated for dimensionless bending rigidities of 10^-5 ≤ K_b ≤ 10^-1 and dimensionless stretching coefficients of 0.1 ≤ K_s ≤1. It was found that while at higher bending rigidities the filaments simply act as roughness elements, at lower bending rigidities and for appropriate heights and spacings of the filaments, the filaments can interact with the near-wall vortical turbulence structures to result in a net drag reduction. The mechanism of drag reduction will be discussed